Method of recording identifier and set of photomasks

ABSTRACT

A method of recording different identifiers, each including at least one character, on multiple plate-type members, involves the use of a photomask of a first type and at least two photomasks of a second type. The photomask of the first type has an opaque pattern that defines a blank region to write the identifier thereon. Each of the photomasks of the second type has an opaque pattern defining the at least one character. The method further includes the steps of forming a photoresist layer on the surface of one of the plate-type members, exposing the photoresist layer, except the blank region, to a radiation through the photomask of the first type, and forming a latent image of the at least one character in the blank region through at least one of the photomasks of the second type.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of recording anidentifier (i.e., an ID or an ID mark) on a plate member and a set ofphotomasks for use in such a method. Preferred embodiments of thepresent invention are effectively applicable for use to inscribe anidentifier not only on a ceramic wafer for a magnetic head, for example,but also on a semiconductor wafer or any other plate-type member.

[0003] 2. Description of the Related Art

[0004] Recently, a thin-film magnetic head having any of variousstructures often includes a magnetic head slider for use in a hard diskdrive (HDD), a tape storage and a flexible (or floppy) disk drive (FDD),for example. Examples of wafers for such a thin-film magnetic headinclude sintered wafers having compositions such as Al₂O₃—TiC, SiC andZrO₂.

[0005]FIG. 1A illustrates a typical thin-film magnetic head slider 10.On its tracking side, this magnetic head slider 10 includes two siderails 11 that will be opposed to the surface of a magnetic disk. Thesurface of the thin-film magnetic head slider 10 on which the side rails11 are provided is sometimes called an “air bearing surface (ABS)”. Ifthe magnetic disk is rotated at a high velocity by a motor, for example,while the surface of the magnetic disk is pressed lightly by the siderails 11 of the magnetic head slider 10 by way of a head suspension,then an air layer will be formed on the surface of the magnetic disk andwill reach the back surface of the air bearing surface of the slider 10.As a result, the magnetic head slider 10 is slightly lifted up. In thismanner, the magnetic head slider 10 can perform read and writeoperations on the magnetic disk while “flying” near the surface of thedisk so to speak.

[0006] A thin film 12, which causes a magnetic interaction with astorage medium such as a magnetic disk, is deposited on one end surfaceof the magnetic head slider 10. The thin film 12 is used to form part ofan electrical/magnetic transducer. To indicate the type of the product,an identifier (ID or ID mark) 13 such as a serial number is inscribed onthe other end surface of the magnetic head slider 10. Methods ofinscribing an identifier 13 on sintered wafers are disclosed in JapaneseLaid-Open Publications Nos. 9-81922, 10-134317 and 11-126311, forexample.

[0007] In a typical manufacturing process, the magnetic head slider 10is obtained by cutting out a bar 20 shown in FIG. 1B from a sintered 1shown in FIG. 1C and then dicing the bar 20 into a great number ofchips. In FIG. 1C, the end surface 4 of the wafer 1 is parallel to theair bearing surface of the magnetic head slider 10 shown in FIG. 1A.

[0008] Recently, as the sizes of such a thin-film magnetic head havebeen decreased to reduce the sizes and weight of an electronicappliance, the thickness of the wafer 1 (corresponding to the length Lof the magnetic head slider 10) and the thickness T of each bar 20(corresponding to the height of the magnetic head slider 10) have alsobeen reduced. For example, a magnetic head slider, which is called a“pico-slider”, has a length L of about 1.2 mm and a thickness T of about0.3 mm. As for a magnetic head slider of such drastically reduced sizes,the sizes of characters to be inscribed on the slider should also bereduced correspondingly.

[0009] In the prior art, a laser marking method is often used toinscribe the identifier 13. In the laser marking method, the identifiers13 shown in FIGS. 1A and 1B are written on the back surface 3 of thewafer 1 that is yet to be divided into the bars 20. After the ID markingprinting process step is finished, various thin films 12 are stacked onthe surface 2 of the wafer 1.

[0010] Hereinafter, the conventional laser marking method will bedescribed briefly with reference to FIG. 2.

[0011] In the laser marking method, the back surface 3 of the wafer 1 islocally irradiated with a laser beam 6 that has been condensed by a lens5, thereby rapidly heating and vaporizing the irradiated portion of thewafer 1. In this case, a tiny concave portion is formed on the backsurface 3 of the wafer 1, while the material of the sintered wafer 1 isscattered around and just a portion of the scattered material isdeposited on the wafer 1 again. By scanning the back surface 3 of thewafer 1 with the laser beam 6, the concave portions can be arranged soas to form an arbitrary pattern on the back surface 3 (which will beherein referred to as a “concave pattern”). Any of various types ofidentifiers 13 can be written at an arbitrary location on the wafer 1 byforming a concave pattern, which is made up of alphanumeric and/ornumeric characters or a barcode, on the back surface 3 of the wafer 1.

[0012] A laser marking method as described above, however, has thefollowing drawbacks.

[0013] Firstly, the portion of the sintered material that has beenscattered around as a result of the exposure to the laser beam is likelyadsorbed or deposited as dust onto the inscribed characters, thuscausing a contamination problem in many cases.

[0014] Secondly, the edges of the inscribed characters are often burredthrough the exposure to the laser beam. Thus, a deburring processingstep needs to be carried out.

[0015] A photolithographic method was used as an alternative identifiermarking method that can avoid these problems. In the photolithographicprocess, first, the back surface 3 of the wafer 1 is coated with aphotoresist layer. Next, the photoresist layer is exposed to a radiationthrough a particular photomask, thereby forming a latent image of theidentifier in a desired region of the photoresist layer. Thereafter, theexposed photoresist layer is developed so as to transfer a patternrepresenting the identifier onto the photoresist layer. Finally, thewafer is selectively etched away by using the patterned photoresistlayer as an etching mask. In this manner, the identifier can be writtenon the wafer. According to this photolithographic process, a fine-linepattern can be formed and any of various types of identifiers can beclearly recorded even within a narrow region.

[0016] The conventional photolithographic process described above is farfrom being cost effective for the following reasons. Specifically, whendifferent types of identifiers should be written on multiple wafers,this photolithographic process requires the same number of photomasks asthat of the wafers. However, photomasks are normally expensive. Forexample, just one photomask sometimes costs hundreds of thousand yen(i.e., over $1,000). For that reason, as the number of photomasks neededincreases, the overall processing cost of this photolithographic processincreases by leaps and bounds, thus constituting a great obstacle todesired cost reduction.

SUMMARY OF THE INVENTION

[0017] In order to overcome the problems described above, preferredembodiments of the present invention provide a method of recordingvarious types of identifiers on a number of plate-type members by usinga much smaller number of photomasks as compared to conventional methods.

[0018] Preferred embodiments of the present invention also provide anovel exposure process that is effectively applicable for use in theidentifier recording method and further provide a set of photomasks foruse with the exposure process.

[0019] A preferred embodiment of the present invention provides a methodof recording mutually different identifiers on multiple plate members.Each of the identifiers preferably includes at least one character. Themethod preferably includes the step of preparing a photomask of a firsttype and at least two photomasks of a second type. The photomask of thefirst type preferably has an opaque pattern that defines a blank regionon which the identifier should be written. Each of the at least twophotomasks of the second type preferably has an opaque pattern thatdefines the at least one character. The method preferably furtherincludes the steps of forming a photoresist layer on the surface of oneof the plate-type members, exposing all of the photoresist layer but theblank region to a radiation by using the photomask of the first type,and forming a latent image of the at least one character in the blankregion of the photoresist layer by using at least one of the photomasksof the second type.

[0020] In one preferred embodiment of the present invention, each of theidentifiers is preferably a character string including at least twocharacters. The step of forming the latent image of the at least onecharacter preferably includes the steps of performing first and secondexposure processes. In the first exposure process, the latent image ofone of the at least two characters is preferably formed in a firstportion of the blank region of the photoresist layer by using one of thephotomasks of the second type. In the second exposure process, thelatent image of another or the other one of the at least two charactersis preferably formed in a second portion of the blank region of thephotoresist layer by using the photomask of the second type that is thesame as, or different from, the photomask used in the first exposureprocess, thereby forming the character string including the at least twocharacters in the photoresist layer.

[0021] Another preferred embodiment of the present invention provides amethod of recording mutually different identifiers on multipleplate-type members. Each of the identifiers is preferably a group ofcharacter strings including character strings A and B. The characterstring A preferably includes of a number z₁ of characters and thecharacter string B preferably includes a number z₂ of characters, wherez₁ and z₂ are both natural numbers. The method preferably includes thestep of preparing a photomask of a first type and at least one photomaskof a second type. The photomask of the first type preferably has opaquepatterns that define a blank region, on which the character string Bshould be written, and the character string A. The at least onephotomask of the second type preferably has an opaque pattern thatdefines at least one of the characters of the character string B. Themethod preferably further includes the steps of forming a photoresistlayer on the surface of one of the plate-type members, selectivelyexposing the photoresist layer to a radiation by using the photomask ofthe first type, thereby defining an unexposed region, corresponding tothe blank region, and a latent image of the character string A in thephotoresist layer, and forming a latent image of the at least onecharacter of the character string B in the unexposed region of thephotoresist layer by using the at least one photomask of the secondtype.

[0022] In one preferred embodiment of the present invention, thecharacter string B preferably includes at least two characters. The stepof forming the latent image of the at least one character preferablyincludes the step of forming the latent image of one of the at least twocharacters of the character string B after another in the unexposedregion of the photoresist layer.

[0023] In another preferred embodiment of the present invention, thestep of forming the latent image of the at least one characterpreferably includes the step of forming the latent image of one of thecharacters of the character string B after another in the unexposedregion of the photoresist layer by using a plurality of photomasks ofthe second type, which includes the at least one photomask of the secondtype, one by one. The photomasks of the second type preferably haveopaque patterns that define mutually different types of characters.

[0024] In still another preferred embodiment, the step of forming thelatent image of the at least one character preferably includes the stepof forming the latent image of one of the characters of the characterstring B after another in the unexposed region of the photoresist layerby using the at least one photomask of the second type having opaquepatterns that define multiple types of characters on the same plane.

[0025] In yet another preferred embodiment, the step of forming thephotoresist layer may include the step of forming the photoresist layerof a negative photoresist material on the surface of the plate-typemember. In that case, the method preferably further includes the step ofdeveloping the photoresist layer of the negative photoresist materialand removing portions of the photoresist layer, which correspond to thecharacter strings, from the surface of the plate-type member.

[0026] In yet another preferred embodiment, the step of forming thephotoresist layer may include the step of forming the photoresist layerof a positive photoresist material on the surface of the plate-typemember. In that case, the method preferably further includes the step ofdeveloping the photoresist layer of the positive photoresist materialand leaving portions of the photoresist layer, which correspond to thecharacter strings, on the surface of the plate-type member.

[0027] Still another preferred embodiment of the present inventionprovides a method of recording mutually different identifiers onmultiple plate-type members. Each of the identifiers is preferably agroup of character strings including character strings A and B. Thecharacter string A preferably includes a number z₁ of characters and thecharacter string B preferably includes a number z₂ of characters, wherez₁ is an integer that is substantially equal to or greater than zero andz₂ is a natural number. The method preferably includes the step ofpreparing multiple photomasks of a first type and multiple photomasks ofa second type. Multiple opaque patterns, defining blank regions on eachof which the character string B should be written and the characterstrings A, are preferably arranged periodically in X-axis and Y-axisdirections on each of the photomasks of the first type. On the otherhand, multiple opaque patterns, each defining one of the characters ofthe character string B, are preferably arranged periodically in theX-axis and Y-axis directions on each of the photomasks of the secondtype. The method preferably further includes the steps of forming aphotoresist layer on the surface of one of the plate-type members,selectively exposing the photoresist layer to a radiation by using oneof the photomasks of the first type, thereby defining unexposed regions,corresponding to the blank regions, and latent images of the characterstrings A in the photoresist layer, and forming latent images of one ofthe characters of the character string B after another in the unexposedregions of the photoresist layer by sequentially selecting a requiredone of the photomasks of the second type after another.

[0028] In one preferred embodiment of the present invention, the step ofpreparing the photomasks of the first and second types preferablyincludes the step of arranging m different types of characters of thecharacter strings B within a portion of at least one of the photomasksof the second type. The portion preferably has a length of y2 asmeasured in the Y-axis direction, where y2 is a pitch between twocharacters of the same type on each of the photomasks of the secondtype. An inequality m≦y2/y1 is preferably satisfied, where y1 is a sizeof the characters as measured in the Y-axis direction on each of thephotomasks of the second type.

[0029] In another preferred embodiment of the present invention, thestep of forming the latent images of the characters of the characterstring B preferably includes the steps of aligning a first one of thephotomasks of the second type with the photoresist layer so that a firstselected character on the first photomask of the second type isprojected onto an associated portion of the unexposed region. The stepof forming the latent images preferably further includes the step ofperforming a first exposure process to form a latent image of the firstselected character in the unexposed region of the photoresist layerafter the step of aligning the first photomask of the second type withthe photoresist layer has been performed. The step of forming the latentimages preferably further includes the step of aligning either the firstphotomask of the second type or a second one of the photomasks of thesecond type with the photoresist layer so that a second selectedcharacter on the first or second photomask of the second type isprojected onto another associated portion of the unexposed region. Thestep of forming the latent images preferably further includes the stepof performing a second exposure process to form a latent image of thesecond selected character in the unexposed region of the photoresistlayer after the step of aligning the first or second photomask of thesecond type with the photoresist layer has been performed.

[0030] In this particular preferred embodiment, the method preferablyfurther includes the step of forming alignment marks before the step offorming the photoresist layer is performed. The step of aligning thefirst photomask of the second type and/or the step of aligning the firstor second photomask of the second type are/is preferably carried out byusing the alignment marks.

[0031] In still another preferred embodiment, the steps of forming thelatent images preferably include the step of forming the latent imagesof the character strings over the entire surface of the photoresistlayer by using a mask aligner.

[0032] Alternatively, the steps of forming the latent images preferablyinclude the step of forming the latent images of one of the characterstrings after another in multiple divided regions of the photoresistlayer by using a stepper.

[0033] Yet another preferred embodiment of the present inventionprovides a method of recording mutually different identifiers onmultiple plate-type members. Each of the identifiers is preferably agroup of character strings including character strings A, B and C. Thecharacter string A preferably includes a number z₁ of characters, thecharacter string B preferably includes a number z₂ of characters, andthe character string C preferably includes a number z₃ of characters,where z₁ is an integer that is substantially equal to or greater thanzero and z₂ and z₃ are both natural numbers. The method preferablyincludes the steps of forming a photoresist layer on the surface of oneof the plate-type members, performing a first exposure process andperforming a second exposure process. The first exposure process ispreferably performed in such a manner that latent images of thecharacter strings A and C are formed in each of multiple subdividedregions of the photoresist layer but that a blank region of each of thesubdivided region of the photoresist layer, on which the characterstring B should be written, is not exposed to a radiation. The secondexposure process is preferably performed in such a manner that latentimages of one of the characters of the character string B are formed inthe blank regions of the multiple subdivided regions of the photoresistlayer and then latent images of another character of the characterstring B are formed in the blank regions.

[0034] In one preferred embodiment of the present invention, the step ofperforming the first exposure process preferably includes the step offorming the latent images of mutually different character strings C inthe multiple subdivided regions of the photoresist layer.

[0035] In this particular preferred embodiment, the multiple subdividedregions of the photoresist layer are preferably arranged in columns androws. Supposing the character string C that is allocated to one of themultiple subdivided regions is located at an intersection between anM^(th) row and an N^(th) column (where M and N are both natural numbers)and is identified by C_(MN), M≠J and/or N≠K (where J and K are bothnatural numbers), C_(MN)≠C_(JK) is preferably satisfied.

[0036] More particularly, supposing the character string A that isallocated to one of the multiple subdivided regions is located at theintersection between the M^(th) row and the N^(th) column (where M and Nare both natural numbers) and is identified by A_(MN), MT≠J and/or N≠K(where J and K are both natural numbers), A_(MN)=A_(JK) is preferablysatisfied.

[0037] In another preferred embodiment of the present invention, thestep of performing the first exposure process preferably includes thestep of exposing the photoresist layer to the radiation by using atleast one photomask of a first type having opaque patterns that definethe character strings A and C. The step of performing the secondexposure process preferably includes the step of exposing thephotoresist layer to the radiation by using at least one photomask of asecond type having opaque patterns that define the characters of thecharacter string B.

[0038] In yet another preferred embodiment, the step of performing thefirst exposure process preferably includes the step of exposing thephotoresist layer to the radiation by using at least one photomask of afirst type having opaque patterns that define the character strings Aand C and opaque patterns that define unexposed regions for thecharacter strings B. The step of performing the second exposure processpreferably includes the step of exposing the photoresist layer to theradiation by using at least one photomask of a second type having opaquepatterns that define the characters of the character strings B.

[0039] In yet another preferred embodiment, the step of performing thefirst exposure process preferably includes the step of forming thelatent images of the character strings A and C over the entire surfaceof the photoresist layer by using a mask aligner.

[0040] In yet another preferred embodiment, the step of performing thesecond exposure process preferably includes the step of forming thelatent images of the character strings B in one of multiple dividedregions of the photoresist layer after another by using a stepper. Eachof the multiple divided regions of the photoresist layer is preferablywider than each of the multiple subdivided regions of the photoresistlayer and preferably includes the multiple subdivided regions of thephotoresist layer.

[0041] In this particular preferred embodiment, the step of forming thelatent images of the character strings B preferably includes the step offorming the latent images of one type of character strings B in one ofthe divided regions of the photoresist layer and then forming the latentimages of another type of character strings B in another one of thedivided regions of the photoresist layer.

[0042] Yet another preferred embodiment of the present inventionprovides a method of patterning a photoresist layer. The methodpreferably includes the steps of preparing the plate-type memberincluding the photoresist layer that has been exposed to the radiationby the method according to any of the preferred embodiments of thepresent invention described above, and developing the photoresist layer.

[0043] Yet another preferred embodiment of the present inventionprovides a method of recording an identifier on a plate-type member. Themethod preferably includes the steps of preparing the plate-type memberincluding the photoresist layer that has been patterned by thepatterning method according to the preferred embodiment described above,and transferring the pattern of the identifier onto the surface of theplate-type member by using the patterned photoresist layer as a mask.

[0044] In one preferred embodiment of the present invention, the step oftransferring the pattern of the identifier may include the step ofselectively etching away surface portions of the plate-type member thatare not covered with the patterned photoresist layer.

[0045] Alternatively, the step of transferring the pattern of theidentifier may include the step of forming convex portions on surfaceregions of the plate-type member that are not covered with the patternedphotoresist layer.

[0046] As another alternative, the step of transferring the pattern ofthe identifier may include the step of altering surface regions of theplate-type member that are not covered with the patterned photoresistlayer by exposing the surface regions to an energy beam.

[0047] Yet another preferred embodiment of the present inventionprovides a method of manufacturing electronic components. The methodpreferably includes the steps of preparing the plate-type member onwhich the identifiers have been recorded by the method according to anyof the preferred embodiments of the present invention described above,depositing a thin film on a selected surface of the plate-type member,and dicing the plate-type member into multiple divided elements.

[0048] In one preferred embodiment of the present invention, the methodpreferably further includes the step of recording the identifiers on theplate-type member so that each of the divided elements has one of theidentifiers on the surface thereof.

[0049] In this particular preferred embodiment, the step of recordingthe identifiers preferably includes the step of recording theidentifiers on the plate-type member so that each of the dividedelements has a unique identifier on the surface thereof.

[0050] Yet another preferred embodiment of the present inventionprovides a set of photomasks for use to transfer the pattern of anidentifier, including a character string that includes at least twocharacters, onto a photoresist layer. The set preferably includes aphotomask of a first type including an opaque pattern that defines ablank region on which the character string should be written and atleast one photomask of a second type including opaque patterns thatdefine the at least two characters of the character string.

[0051] Yet another preferred embodiment of the present inventionprovides a set of photomasks for use to transfer the pattern of anidentifier onto a photoresist layer. The identifier is preferably agroup of character strings including character strings A and B. Thecharacter string A preferably includes a number z₁ of characters and thecharacter string B preferably includes a number z₂ of characters, wherez₁ is an integer that is substantially equal to or greater than zero andz₂ is a natural number. The set preferably includes a plurality ofphotomasks of a first type and a plurality of photomasks of a secondtype. Multiple opaque patterns, defining blank regions on each of whichthe character string B should be written and the character strings A,are preferably arranged periodically in X-axis and Y-axis directions oneach of the photomasks of the first type. Multiple opaque patterns, eachdefining one of the characters of the character string B, are preferablyarranged periodically in the X-axis and Y-axis directions on each of thephotomasks of the second type.

[0052] In one preferred embodiment of the present invention, m differenttypes of characters of the character strings B are preferably arrangedwithin a portion of at least one of the photomasks of the second type.The portion preferably has a length of y2 as measured in the Y-axisdirection, where y2 is a pitch between two characters of the same typeon each of the photomasks of the second type. An inequality m≦y2/y1 ispreferably satisfied, where y1 is a size of the characters as measuredin the Y-axis direction on each of the photomasks of the second type.

[0053] In another preferred embodiment of the present invention,multiple opaque patterns, defining character strings C, are preferablyarranged periodically in the X-axis and Y-axis directions on each of thephotomasks of the first type. Each of the character strings C preferablyincludes a number z₃ of characters, where z₃ is a natural number.

[0054] Other features, elements, processes, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of preferred embodiments of the presentinvention with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055]FIG. 1A is a perspective view of a slider for a magnetic head.

[0056]FIG. 1B is a perspective view illustrating a bar yet to be dividedinto multiple sliders for a magnetic head.

[0057]FIG. 1C is a perspective view illustrating a substantiallyrectangular sintered wafer.

[0058]FIG. 2 schematically illustrates a conventional laser markingprocess.

[0059]FIG. 3A shows an exemplary identifier.

[0060]FIG. 3B shows a situation where multiple different types ofidentifiers are recorded on multiple wafers so that identifiers of thesame type are written at multiple locations on each wafer.

[0061]FIG. 3C shows a situation where multiple different types ofidentifiers are recorded on multiple wafers so that a single identifieris written at one location on each wafer.

[0062]FIG. 4A schematically illustrates a configuration for photomasksof a first type according to a first specific preferred embodiment ofthe present invention.

[0063]FIG. 4B schematically illustrates a configuration for photomasksof a second type according to the first preferred embodiment of thepresent invention.

[0064]FIG. 5A is a plan view illustrating a portion of a photomask ofthe first type according to the first preferred embodiment of thepresent invention.

[0065]FIGS. 5B and 5C are cross-sectional views illustrating a firstexposure process that is carried out by using the photomask of the firsttype shown in FIG. 5A.

[0066]FIG. 6A is a plan view illustrating a portion of a photomask ofthe second type according to the first preferred embodiment of thepresent invention.

[0067]FIGS. 6B and 6C are cross-sectional views illustrating a secondexposure process that is carried out by using the photomask of thesecond type shown in FIG. 6A.

[0068]FIG. 7A is a plan view illustrating a portion of another photomaskof the second type according to the first preferred embodiment of thepresent invention.

[0069]FIGS. 7B and 7C are cross-sectional views illustrating the secondexposure process that is carried out by using the photomask of thesecond type shown in FIG. 7A.

[0070]FIG. 7D is a plan view illustrating a photoresist layer that hasbeen subjected to the first and second exposure processes.

[0071]FIGS. 8A through 8E are cross-sectional views illustrating variousmethods of recording an identifier on a wafer including a developedphotoresist layer thereon, wherein:

[0072]FIG. 8A illustrates the wafer including the photoresist layer thathas just been developed.

[0073]FIG. 8B illustrates the wafer that has been subjected to anetching process.

[0074]FIGS. 8C and 8D illustrate a lift-off process.

[0075]FIG. 8E illustrates a surface altering process.

[0076]FIG. 9A illustrates a configuration for a photomask of a secondtype for use in a second specific preferred embodiment of the presentinvention.

[0077]FIG. 9B illustrates identifiers that have been recorded accordingto the second preferred embodiment of the present invention.

[0078]FIG. 10A schematically illustrates one character A1 and atwo-digit blank region that have been written on a photoresist layeraccording to the second preferred embodiment of the present invention.

[0079]FIG. 10B is a plan view illustrating a state where Arabic numerals“0” are being written as characters B1 on the photoresist layeraccording to the second preferred embodiment of the present invention.

[0080]FIG. 10C is a plan view illustrating a state where Arabic numerals“2” are being written as characters B2 on the photoresist layeraccording to the second preferred embodiment.

[0081]FIG. 11A illustrates an identifier for use in a third specificpreferred embodiment of the present invention.

[0082]FIG. 11B is a plan view illustrating a wafer on which identifiershave been recorded according to the third preferred embodiment of thepresent invention.

[0083]FIG. 11C is plan view illustrating a configuration for a photomaskof a first type for use in the third preferred embodiment of the presentinvention.

[0084]FIG. 12 is a plan view schematically illustrating the arrangementof character string groups for use in a fourth specific preferredembodiment of the present invention.

[0085]FIGS. 13A and 13B are plan views illustrating how alignment marksmay be laid out in a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0086] Hereinafter, a first specific preferred embodiment of the presentinvention will be described with reference to the accompanying drawings.In the first preferred embodiment, mutually different identifiers arerecorded on multiple ceramic wafers.

[0087] In this preferred embodiment, the identifier to be recorded oneach of the wafers is represented by a group of character stringsincluding a character string A including a number z₁ of characters and acharacter string B including a number z₂ of characters, where z₁ and z₂are both natural numbers. In the following illustrative preferredembodiment, the character string A is supposed to be a one-digitcharacter string consisting of just one character A1 and the characterstring B is supposed to be a two-digit character string including twocharacters B1 and B2 as shown in FIG. 3A for the sake of simplicity.Also, each of these characters A1, B1 and B2 is preferably one of theten Arabic numerals of 0 through 9. That is to say, 1,000 differenttypes of identifiers (i.e., “000” through “999”) can be represented bythe combinations of these character strings A and B. In this preferredembodiment, each of the characters included in each character string issupposed to be an Arabic numeral for illustrative purposes only.However, those characters may also be alphabets, Japanese syllabicscripts (“hiragana” or “katakana”), Chinese characters (“kanji”),symbols or whatever belongs to a coding unit that can represent theidentifier. It should also be noted that although a character stringincluding a number n of characters (where n is a natural number) will beherein referred to as an “n-digit character string”, not all of thesecharacters have to be Arabic numerals but some of them may be a symbolof one of those types. Therefore, the “digit” does not herein alwaysmean a numeral.

[0088] The identifiers AB are preferably inscribed at multiple locationsof each wafer as shown in FIG. 3B. Optionally, the identifier AB mayalso be written at just one location of each wafer as shown in FIG. 3C.If the identifiers are recorded on the wafers as shown in FIG. 3C, theidentifiers are effective while various manufacturing and processingsteps are carried out on a wafer-by-wafer basis. However, once eachwafer has been diced into multiple chips, the identifiers are no longereffective.

[0089] To inscribe 1,000 different types of identifiers on 1,000 wafersby the conventional photolithographic process, 1,000 photomasks (orreticles) are needed. In that case, mutually different identifiersshould be allocated to the respective photomasks. For example, an opaquepattern defining a character string “150” is formed on a 150^(th)photomask.

[0090] In contrast, according to this preferred embodiment, a negativephotoresist layer is subjected to multiple exposure processes by using aset of photomasks including two types of photomasks (i.e., photomasks ofa first type and photomasks of a second type). In this manner, theidentifiers required can be inscribed on the same number of wafers byusing a far smaller number of photomasks.

[0091] Hereinafter, photomasks for use in this preferred embodiment willbe described with reference to FIGS. 4A and 4B.

[0092] First, referring to FIG. 4A, illustrated are photomasks of thefirst type for use in this preferred embodiment. Each of the photomasksof the first type includes multiple opaque patterns each defining ablank region on which the character string B should be written andmultiple opaque patterns each defining the character string A. Inwriting multiple different types of identifiers on multiple wafers sothat the identifiers of the same type are recorded on a plurality ofregions of each wafer as shown in FIG. 3B, the opaque patterns definingthe character strings A and the blank regions are arranged periodicallyin the X-axis and Y-axis directions on each photomask of the first type.In the preferred embodiment illustrated in FIG. 4A, the character A1 andthe two-digit blank region are arranged from left to right on eachregion on which the identifier should be written. In this specificexample, the character A1 is one of the Arabic numerals of “0” through“9”, thus requiring ten different photomasks of the first type. If eachcharacter string A consists of two digits, 100 different photomasks ofthe first type will be needed to represent 100 types of characterstrings of “00” through “99”.

[0093] On each of the photomasks of the second type on the other hand,patterns each defining one of the characters of the character string Bare preferably arranged periodically in the X-axis and Y-axis directionsas shown in FIG. 4B. The pitch between two adjacent one-digit charactersis substantially equal to the pitch between two adjacent characterstrings A. Each of the photomasks of the second type is disposed so thateach character on the photomask of the second type is aligned with oneof the two portions of its associated two-digit blank region on itsassociated photomask of the first type.

[0094] Hereinafter, it will be described with reference to FIGS. 5Athrough 7D how to carry out exposure processes using the set ofphotomasks.

[0095] In this specific example, a photomask of the first type 62 shownin FIG. 5A is used. On this photomask 62, an opaque pattern 62 adefining a character A1 and an opaque pattern 62 b defining a two-digitblank region have been formed. A photomask like this can be obtained bydefining these patterns of a metal or resin film having opacity on atransparent substrate of glass, for example. FIG. 5A illustrates onlythe opaque patterns 62 a and 62 b for just one identifier for the sakeof simplicity. Actually, though, a great number of similar opaquepatterns 62 a and 62 b are arranged on the single photomask 62.

[0096] In the specific example shown in FIG. 5A, the opaque pattern 62 bdefining the two-digit blank region is a single continuous portion of afilm. Alternatively, the opaque pattern 62 b may be divided into twoportions corresponding to the two characters. In any case, the opaquepattern 62 b preferably has sizes and a shape that are sufficientlygreater than those of the character string B to be written thereon. Thesizes of the opaque pattern 62 b should be designed appropriately withthe potential back-reflection of diffracted light and potentialshrinkage and/or expansion of the resist patterns taken into account.

[0097] Next, as shown in FIG. 5B, a wafer 60 that has been coated with anegative photoresist layer 61 is prepared. The photoresist layer 61 onthe wafer 60 is exposed to a radiation through the photomask 62 of thefirst type. As a result of this exposure process, latent images of thepatterns 62 a and 62 b, which define the character A1 and the two-digitblank region, respectively, are formed in their associated portions ofthe photoresist layer 61. More specifically, the radiation is not passedthrough the character A1 representing one of the Arabic numerals of 0through 9 (e.g., “1” in the specific example illustrated in FIG. 5A) orthe two-digit blank region. Accordingly, the photoresist layer 61 ispartially exposed to the radiation that has been transmitted through theremaining portions of the photomask 62 other than the opaque patterns 62a and 62 b. In FIG. 5C, those exposed portions of the photoresist layer61 are indicated by dots. Since a negative photoresist material is usedin this specific example, the unexposed portions of the negativephotoresist layer 61 will be dissolved in a developer and form openingsin a developing process step to be performed later. As a result,portions of the photoresist layer 61 corresponding to the characterswill be removed in this preferred embodiment.

[0098] It should be noted that an aligner is preferably used in thisfirst exposure process to form latent images defining the characterstrings A and the blank regions over the entire surface of the wafer 60.

[0099] Subsequently, before the developing process step is performed,the character string B is written on the blank region. Specifically, anappropriate photomask of the second type is selected from the tenphotomasks of the second type shown in FIG. 4B. By using the photomaskselected, the latent images of the characters B1 are formed in thephotoresist layer 61. FIG. 6A illustrates a portion of the photomask 63selected. As shown in FIG. 6A, the photomask 63 of the second typeincludes an opaque pattern 63 a defining the character of the characterstring B and an opaque pattern 63 b that cuts off unnecessary light.

[0100] By aligning the photomask 63 of the second type with thephotoresist layer 61 appropriately, the character B1 can be aligned withone of the two portions of the blank region, thereby exposing theselected portion of the blank region to the radiation as shown in FIG.6B. The, blank region of the photoresist layer 61 was not exposed to theradiation in the process step shown in FIG. 5B. Accordingly, when thatportion of the blank region is exposed to the radiation that has passedthrough the pattern 63 a defining the character B1, a latent imagecorresponding to that pattern 63 a is formed in the blank region asshown in FIG. 6C. To prevent the other unexposed portions of thephotoresist layer 61 (i.e., a portion corresponding to the latent imageof the character A1 and the other portion of the blank region) frombeing exposed to the radiation unnecessarily, the opaque pattern 63 b ofthe photomask 63 cuts off the unnecessary light.

[0101] Next, a third exposure process is carried out as shown in FIGS.7A through 7D. When the second exposure process shown in FIGS. 6Athrough 6C is finished, another appropriate photomask 64 of the secondtype is selected from the ten photomasks of the second type shown inFIG. 4B. By using the photomask 64 selected, the latent image of thecharacter B2 is formed in the photoresist layer 61. As shown in FIG. 7A,the photomask 64 of the second type includes an opaque pattern 64 adefining the character of the character string B and an opaque pattern64 b that cuts off unnecessary light. By aligning the photomask 64 withthe photoresist layer 61 appropriately, the character B2 can be alignedwith the other unexposed portion of the blank region, thereby exposingthat portion of the blank region to the radiation as shown in FIG. 7B.Accordingly, when that portion of the blank region is exposed to theradiation that has passed through the pattern defining the character B2,a latent image corresponding to that pattern is formed in the blankregion as shown in FIG. 7C.

[0102] By performing these exposure processes, the identifier shown inFIG. 7D is written on each of multiple regions of the negativephotoresist layer 61. Thereafter, when the negative photoresist layer 61is subjected to the developing process, the unexposed portions (i.e.,the non-dotted white portions) are removed from the photoresist layer 61and openings that define the intended characters are formed in thephotoresist layer 61. FIG. 8A schematically illustrates a cross sectionof the negative photoresist layer 61 that has been developed.

[0103] Hereinafter, various methods of recording the identifiers on thewafer 60 by using the patterned photoresist layer 61 (which will beherein also referred to as a “resist pattern”) will be described withreference to FIGS. 8B through 8E.

[0104] First, a method utilizing an etching technique will be described.In this case, the wafer 60 including the resist pattern 61 thereon isloaded into an etcher (not shown) so that the surface of the wafer 60reacts with an appropriate etchant or etching gas. Then, surfaceportions of the wafer 60 that are not covered with the resist pattern 61are etched away, thereby transferring the character strings A and B ontothe wafer 60 as shown in FIG. 8B. The conditions of this etching processare appropriately selected depending on the material of the wafer 60 tobe etched. The etching process may be either a dry etching process suchas a plasma-enhanced reactive ion etching (RIE) process or a wet etchingprocess, or other suitable process.

[0105] The techniques of recording the identifiers on the wafer 60 byusing the developed and patterned photoresist layer 61 are not limitedto the etching process described above. For example, as shown in FIG.8C, a thin film 65 of a metal, for example, may be deposited over thewafer 60 that is partially covered with the resist pattern (i.e., thepatterned photoresist layer) 61 and then the resist pattern 61 may bestripped along with the excessive portions of the thin film 65. Even so,the identifiers can also be transferred onto the wafer 60. A techniquesuch as this is called a “lift-off” process. In this technique, theexcessive portions of the thin film 65 that have been deposited on theresist pattern 61 are removed from the surface of the wafer 60 alongwith the resist pattern 61. As a result, the other portions of the thinfilm 65 that were located inside the openings of the resist pattern 61(i.e., portions defining the characters in this preferred embodiment)are left on the surface of the wafer 60, thereby forming raisedcharacter portions that protrude from the surface of the wafer 60 asshown in FIG. 8D.

[0106] The identifiers may also be recorded by any technique other thanthe methods of forming the embossed patterns on the surface of the wafer(including the etching and lift-off processes described above). Forexample, as shown in FIG. 8E, the bare surface portions of the wafer 60that is still covered with the resist pattern 61 may be irradiated with,and modified by, an energy beam such as an ion beam. Even so, theidentifiers can also be transferred onto the wafer 60. When thosesurface portions of the wafer 60 are modified, the reflectance,conductivity and other physical parameters thereof change. Accordingly,by detecting such changes, the identifiers can also be read from thewafer.

[0107] It should be noted that if the wafer 60 is made of a compositematerial including Al₂O₃ and TiC (e.g., Al₂O₃—TiC type ceramic), thewafer 60 is preferably etched selectively by the method that wasdisclosed by the applicant of the present application in the descriptionand the drawings of Japanese Patent Application No. 2000-239431.According to the selective etching process, embossed patterns having avery small depth are formed on the surface of the character portions.Thus, even though the etched portions have a small depth, the differencein reflectance between the character portions and the non-characterportions is so great as to read the identifiers easily.

[0108] It should also be noted that the character string A may beomitted from the identifier. When the character string A is omitted, thenumber z₁ of characters included in the character string A is zero.Accordingly, in this preferred embodiment, each identifier may berepresented by a group of character strings including a character stringA including a number z₁ of characters (where z₁ is an integer that isequal to or greater than zero) and a character string B including anumber z₂ of characters (where z₂ is a natural number).

[0109] If the character string A includes multiple characters (e.g.,characters A1 and A2), those characters A1 and A2 do not have to beadjacent to each other on the wafer. For example, the characters A1 andA2 may interpose the character string B between them. Also, where thecharacter string A is made up of a plurality of characters A1 throughAn, those characters A1 through An do not have to be arranged in line.For example, the characters A1 through An may be arranged eithertwo-dimensionally in columns and rows or to form a curved pattern. Asfor possible arrangements of characters, the same statement is true ofthe character string B.

[0110] Hereinafter, a second specific preferred embodiment of thepresent invention will be described.

[0111] In this preferred embodiment, mutually different identifiers arerecorded on multiple ceramic wafers by using an even smaller number ofwafers.

[0112] In the second preferred embodiment, the same identifiers as thoseof the first preferred embodiment described above are recorded on therespective wafers, and the photomasks of the first type to be used arealso the same as the counterparts of the first preferred embodiment. Thedifference between the first and second preferred embodiments lies inthe structure of the photomasks of the second type.

[0113] In the photomask of the second type for use in this secondpreferred embodiment, multiple different patterns, each defining one ofthe characters of the character string B, are arranged periodically inthe X-axis and Y-axis directions as shown in FIG. 9A. FIG. 9Billustrates two 3-digit character strings B to be formed on a wafer. Thesize of each character to be recorded on a wafer is proportional to thatof its associated character on a photomask. In the same way, a pitchbetween each pair of characters of the same type on a wafer is alsoproportional to a pitch between an associated pair of characters of thesame type on a photomask. In FIG. 9B, the size y1 and the pitch y2,which actually represent the size of each character on a photomask and apitch between each pair of characters on the photomask, respectively,are used to represent the size and pitch of characters on a wafer forconvenience sake. Supposing the size of the characters of as measured inthe Y-axis direction on each photomask of the second type is y1 and thepitch between two characters of the same type as measured in the Y-axisdirection is y2 as shown in FIG. 9B, m different types of characters(where m is a natural number that is equal to or greater than two) ofthe character strings B are arranged within a portion of the photomaskof the second type that has a length of y2 as measured in the Y-axisdirection. In this case, m≦y2/y1 is preferably satisfied.

[0114] More specifically, four types of characters “0” through “3” areallocated to a first photomask of the second type shown in FIG. 9A. Thatis to say, multiple sets of characters “0”, “1”, “2” and “3” arealternately arranged in the Y-axis direction on the first photomask ofthe second type. Another four types of characters “4” through “7” areallocated to a second photomask of the second type (not shown), i.e.,multiple sets of characters “4”, “5”, “6” and “7” are alternatelyarranged in the Y-axis direction on the second photomask of the secondtype. Two other types of characters “8” and “9” are allocated to a thirdphotomask of the second type (not shown), i.e., multiple pairs ofcharacters “8” and “9” are alternately arranged in the Y-axis directionon the third photomask of the second type. In this manner, by usingthese three different photomasks of the second type, the ten types ofcharacters “0” through “9” can be written on the photoresist layer.

[0115] Hereinafter, it will be described with reference to FIGS. 10Athrough 10C specifically how to perform exposure processes in thissecond preferred embodiment of the present invention.

[0116] First, a negative photoresist layer on a wafer is exposed to aradiation by using the photomask of the first type. As a result of thisfirst exposure process, latent images of patterns that define acharacter A1 and a two-digit blank region (which are arranged in thisorder from left to right) are formed in each of multiple regions of thephotoresist layer. FIG. 10A schematically illustrates a state where thecharacter A1 and the two-digit blank region have been written on thephotoresist layer.

[0117] Next, appropriate one is selected from the three differentphotomasks of the second type, and latent images of a character B1 areformed in the photoresist layer by using the selected photomask of thesecond type. In the example illustrated in FIG. 10B, an Arabic numeral“0” is selected as the character B1. In this process step, by aligningthe photomask of the second type with the photoresist layerappropriately, the location of each character B1 is aligned with one ofthe two portions of its associated blank region. Then, that portion ofthe blank region is exposed to the radiation. The portion of the blankregion has not been exposed yet. Accordingly, when the portion of theblank region is exposed to the radiation through the photomask of thesecond type having a pattern that defines the character B1, a latentimage corresponding to the pattern is formed in that portion of theblank region. In this case, to prevent the unexposed portions of thephotoresist layer from being exposed to the radiation unnecessarily, thephotomask of the second type has opaque patterns that block the unwantedradiation. By using the photomask of the second type having such opaquepatterns, no exposing radiation reaches any portion of the photoresistlayer other than the portions on which the characters B1 should bewritten. These opaque patterns should have such sizes and shape as toprevent the unwanted radiation from reaching the portions wherecharacters were written in the previous exposure process or the portionswhere characters will be written in the next exposure process.

[0118] Some of the characters that are arranged in the Y-axis directionon the photomask of the second type overlap with the blank regions. Inthe example illustrated in FIG. 10B, the characters “0” overlap with theblank regions. On the other hand, the radiation that has passed throughthe opaque patterns defining the other characters (i.e., “1”, “2” and“3”) reaches the non-blank regions (i.e., the exposed regions) of thephotoresist layer. Accordingly, no latent images of these characters“1”, “2” and “3” are formed in the photoresist layer. That is to say, itis only on the unexposed blank regions that the characters are writtenin this exposure process.

[0119] Next, an appropriate one is selected from the three differentphotomasks of the second type, and latent images of a character B2 areformed in the photoresist layer by using the selected photomask of thesecond type as shown in FIG. 10C. In the example illustrated in FIG.10C, an Arabic numeral “2” is selected as the character B2. In thissecond exposure process, by aligning the photomask of the second typewith the photoresist layer appropriately, the location of each characterB2 is aligned with the other unexposed portion of its associatedtwo-digit blank region. Then, that portion of the blank region isexposed to the radiation. Accordingly, when the second-digit portion ofthe blank region is exposed to the radiation through the photomask ofthe second type having a pattern that defines the character B2, a latentimage corresponding to the pattern is formed in that portion of theblank region.

[0120] According to this preferred embodiment, multiple types ofcharacters to be included in each character string B can be written onthe photoresist layer by using an even smaller number of photomasks ofthe second type. Thus, the number of photomasks needed can be furtherreduced.

[0121] Hereinafter, a third specific preferred embodiment of the presentinvention will be described.

[0122] In this third preferred embodiment, an identifier to be recordedon each wafer includes a character string B including a number z₂ ofcharacters and a character string C including a number z₃ of characters(where z₂ and z₃ are both natural numbers).

[0123] The third preferred embodiment of the present invention relatesto a method of recording multiple identifiers, including mutuallydifferent character strings C, at various locations on a single wafer.If those different identifiers are provided for respective regions ofone wafer, multiple divided elements, obtained by dicing one wafer asshown in FIGS. 1A through 1C, can have their own identifiers.

[0124] In this specific example, a four-digit identifier is supposed tobe recorded as shown in FIG. 11A for the sake of simplicity. As shown inFIG. 11A, the character string B of this identifier includes twocharacters B1 and B2, while the character string C thereof includes twocharacters C1 and C2. In the example illustrated in FIGS. 11A through11C, each of the characters B1 and B2 may be one of the twenty-sixalphabets of “a” through “z” and each of the characters C1 and C2 may beone of the ten Arabic numerals of “0” through “9”. That is to say,26²×10² different types of identifiers (i.e., “aa00” through “zz99”) canbe represented by combining the character strings B and C in variousmanners. In this case, the two-digit character strings C may be used toprovide mutually different pieces of identification information for 100regions that are arranged in matrix, or in 10 columns and 10 rows. FIG.11B schematically illustrates a wafer plane on which those identifiershave been recorded. If the number of characters per character string Cis increased, it is naturally possible to provide different pieces ofidentification information for an even greater number of wafer regions.

[0125] A character string C, which is allocated to one of the multiplematrix regions that is located at an intersection between an M^(th) rowand an N^(th) column (where M and N are both natural numbers), is hereinidentified by C_(MN). That is to say, C1·C2=C_(MN). In this preferredembodiment, if M≠J and/or N≠K (where J and K are both natural numbers),then C_(MN)≠C_(JK) is satisfied. That is to say, on looking at any givencharacter string C, it can be quickly seen to which column and rowposition of the wafer the identifier including the character string C isallocated. Thus, the character string C will be herein referred to as a“row-column number”. If these identifiers are inscribed on all ofmultiple divided elements of a wafer, then the original location of anydivided part can be specified instantly.

[0126] Next, a photomask for use in this preferred embodiment will bedescribed with reference to FIG. 11C.

[0127] On the photomask of the first type for use in this preferredembodiment, opaque patterns defining two-digit blank regions, on each ofwhich the character string B should be written, and opaque patternsdefining the characters of the character strings C have been formed. Onthe photomask of the first type, the opaque patterns defining thoseblank regions and character strings C are arranged periodically in theX-axis and Y-axis directions.

[0128] The character strings C of the third preferred embodiment aredifferent from the character strings A of the first preferred embodimentdescribed above in that the values of the two-digit character strings Care different from one location on a single wafer to another. However,the combination of the character strings C for one wafer does not haveto be different from that of the character strings C for another wafer.Accordingly, just one photomask is needed to write the character stringsC.

[0129] On the other hand, each photomask of the second type for use inthis third preferred embodiment may have the same structure as thecounterpart of the first or second preferred embodiment described above.Thus, by using the photomask of the second type, the characters “a” and“b” shown in FIG. 11B are written on a digit-by-digit basis on thephotoresist layer.

[0130] Hereinafter, it will be described how to perform exposureprocesses in this third preferred embodiment of the present invention.

[0131] First, a wafer that has been coated with a negative photoresistlayer is prepared, and respective regions of the photoresist layer areexposed to the radiation by using the photomask of the first type. As aresult of this exposure process, latent images of 100 differentcharacter strings C (i.e., “00” through “99”) are formed at respectivelydifferent locations of the photoresist layer, and two-digit blankregions are defined for the respective regions of the photoresist layer.

[0132] Next, an appropriate one is selected from multiple photomasks ofthe second type, and the latent images of the characters B1 are formedin the photoresist layer by using the selected photomask of the secondtype. In this case, by aligning the photomask of the second type withthe photoresist layer appropriately, the location of the character B1 isaligned with one of the two portions of each two-digit blank region.Then, that portion of the blank region is exposed to the radiation.

[0133] Subsequently, an appropriate one is selected from the multiplephotomasks of the second type, and the latent images of the charactersB2 are formed in the photoresist layer by using the selected photomaskof the second type. In this case, by aligning the photomask of thesecond type with the photoresist layer appropriately, the location ofthe character B2 is aligned with the other unexposed portion of eachtwo-digit blank region. Then, that portion of the blank region isexposed to the radiation. When the second-digit portion of the blankregion is exposed to the radiation through the photomask of the secondtype having a pattern that defines the character B2, a latent imagecorresponding to the pattern is formed in that portion of the blankregion.

[0134] In this manner, the character strings B and C are written on therespective regions of the photoresist layer. Thereafter, the photoresistlayer that has been patterned in this manner is developed, therebyremoving the unexposed portions from the photoresist layer. As a result,openings representing the characters are formed in the photoresistlayer.

[0135] Next, the wafer including the resist pattern (i.e., the patternedphotoresist layer) thereon is loaded into an etcher so that the surfaceof the wafer reacts with an appropriate etchant or etching gas. Then,surface portions of the wafer that are not covered with the resistpattern are etched away, thereby transferring the character strings Band C onto the wafer.

[0136] In the third preferred embodiment described above, eachidentifier preferably includes no character string A. Alternatively,each identifier may also include the character string A. In that case,the same photomask of the first type may be used to write the characterstrings A and C on the photoresist layer. However, each of thephotomasks of the first type for use to form the character strings Ashould be provided with opaque patterns defining the character strings Cand opaque patterns defining the blank regions. Optionally, twodifferent photomasks may be used to form the character strings C and toform the character strings A, respectively.

[0137] Hereinafter, a fourth specific preferred embodiment of thepresent invention will be described.

[0138] In this fourth preferred embodiment, each identifier isrepresented by a group of character strings including a character stringA including a number z₁ of characters, a character string B including anumber z₂ of characters and a character string C including a number z₃of characters (where z₁ is an integer equal to or greater than zero andz₂ and z₃ are both natural numbers).

[0139] In this preferred embodiment, a cluster region is formed byassembling a plurality of regions, each including the group of characterstrings A, B and C, as shown in FIG. 12. Multiple cluster regions areallocated to a single wafer so as to have mutually different characterstrings B. That is to say, the character strings B of one type (e.g.,B₁) are included in one cluster region (e.g., cluster region No. 1), thecharacter strings B of another type (e.g., B₂) are included in anothercluster region (e.g., cluster region No. 2), and so on. Morespecifically, an identifier represented by a group of character stringsA_(MN)B_(L)C_(MN) is recorded at the M^(th) row and N^(th) column in theL^(th) cluster region of one wafer, where L is an integer. Among thesecharacter strings A_(MN)B_(L)C_(MN), the character string C_(MN) at aparticular location (i.e., at the M^(th) row and N^(th) column) of onecluster region is the same as the character string C_(MN) at the samelocation (i.e., at the M^(th) row and N^(th) column) of any othercluster region. However, the character strings B_(L) of one clusterregion are different from the character strings B_(L) of another clusterregion. Accordingly, any cluster region can be specified by thecharacter string B_(L), and any location inside a cluster region can bespecified by the character string C_(MN).

[0140] In each cluster region, if M≠J and/or N≠K (where J and K arenatural numbers), then either A_(MN)≠A_(JK) or A_(MN)=A_(JK) may besatisfied. The character strings A represent information to identify thewafer where the character strings A belong. As for the character stringsC_(MN) on the other hand, if M≠J and/or N≠K (where J and K are naturalnumbers), then C_(MN)≠C_(JK) is always satisfied. That is to say, anylocation inside each cluster region is identifiable by the characterstring C_(MN), and therefore, the character string A_(MN) at onelocation inside a cluster region does not always have to be differentfrom the character string A_(MN) at another location inside the samecluster region.

[0141] It should be noted that the order of arrangement of the characterstrings A, B and C and the arrangement of the cluster regions are notlimited to those of the specific example illustrated in FIG. 12.

[0142] In this preferred embodiment, a first exposure process is carriedout by using at least one photomask of the first type so that latentimages of the character strings A and C are formed in respective regionsof a negative photoresist layer but that the blank regions to record thecharacter strings B thereon are not exposed to the radiation.

[0143] Thereafter, a second exposure process is carried out so thatlatent images of the characters of the character strings B are formed ona digit-by-digit basis in the unexposed blank regions. That is to say,the second exposure process preferably includes the same number ofprocess steps as the number of characters per character string B. Thus,each of the character strings B does not have to contain just onecharacter.

[0144] In the first exposure process, the latent images of the characterstrings A and C are preferably formed in the entire photoresist layerover the wafer by using an aligner. In the second exposure process onthe other hand, the latent images of the character strings B arepreferably formed in one cluster region after another by using astepper. The photomask of the second type as described above for thesecond preferred embodiment is preferably used in the second exposureprocess. In that case, by sequentially shifting the photomask of thesecond type from one aligned position to another, latent images ofmultiple types of character strings B can be formed in multipledifferent cluster regions.

[0145] Optionally, the latent images of the character strings A and thelatent images of the character strings C may be formed separately byusing two different photomasks.

[0146] In each of the preferred embodiments of the present inventiondescribed above, a negative photoresist material is used. However, thepresent invention is not limited to those specific preferredembodiments, but can naturally be carried out even by the use of apositive photoresist material. When a positive photoresist material isused, the exposed portions of the positive photoresist layer aredissolved in a developer. Accordingly, if the photomasks described aboveand the positive photoresist material are used in combination, then thecharacter portions (i.e., unexposed portions) of the positivephotoresist layer will remain after the photoresist layer has beendeveloped. And when the wafer is selectively etched by using thepatterned positive photoresist layer as an etching mask, the surface ofwafer is etched entirely except regions under the character portions. Itshould be noted, however, that the patterns of the identifiers may alsobe transferred onto the surface of the wafer without etching the surfaceof the wafer. For example, if a lift-off process is carried out using apatterned positive photoresist layer, then the character portions may berecessed ones.

[0147] Each photomask is preferably aligned with a photoresist layer byreference to an alignment mark on a wafer. For example, an alignmentmark such as that shown in FIG. 13A is preferably formed on the waferbeforehand. An alignment mark like this is preferably a resist pattern.On the other hand, each photomask of the first or second type is alsopreferably provided with another type of alignment marks as shown inFIG. 13B so that one of the alignment marks of the photomask can bealigned with the alignment mark on the wafer. When multiple alignmentmarks are formed on the photomask as shown in FIG. 13B, the maskalignment process can be performed highly accurately.

[0148] Various preferred embodiments of the present invention areeffectively applicable for use not only in a ceramic wafer but also in aplate-type member of any other material. Also, although the plate-typemember of the preferred embodiments described above preferably has theshape of a wafer, the plate-type member may also be a member having anyother arbitrary shape as long as the member has a surface having twodimensions that are large enough to write the identifiers thereon. Thesurface on which the identifiers are written does not have to be a flatsurface but may also be a curved surface. Furthermore, the identifiersare preferably written on flat regions, but not the entire surface ofthe plate-type member has to be flat. Accordingly, some sort of unevenor stepped portions may preexist on the remaining regions of theplate-type member other than the regions on which the identifiers shouldbe written.

[0149] Furthermore, the process step of recording the identifiers on aceramic wafer or a silicon wafer does not have to be performed beforethe process steps of depositing various thin films on the wafer arestarted. Alternatively, the identifiers may also be recorded on thewafer either during a series of thin film deposition and patterningprocess steps or at an arbitrary point in time after those process stepsare finished.

[0150] According to various preferred embodiments of the presentinvention described above, a variety of identifiers can be recorded onmultiple plate-type members by using a much smaller number ofphotomasks. Thus, the unwanted increase in manufacturing cost due to theuse of many expensive photomasks is avoidable.

[0151] Thus, various preferred embodiments of the present inventiondescribed above are effectively applicable for use in a manufacturingprocess that includes the process step of dicing a plate-type membersuch as a wafer into multiple divided elements.

[0152] While the present invention has been described with respect topreferred embodiments thereof, it will be apparent to those skilled inthe art that the disclosed invention may be modified in numerous waysand may assume many embodiments other than those specifically describedabove. Accordingly, it is intended by the appended claims to cover allmodifications of the invention that fall within the true spirit andscope of the invention.

What is claimed is:
 1. A method of recording mutually differentidentifiers on multiple plate members, each said identifier including atleast one character, the method comprising the steps of: (a) preparing aphotomask of a first type and at least two photomasks of a second type,the photomask of the first type having an opaque pattern that defines ablank region on which the identifier should be written, each of the atleast two photomasks of the second type having an opaque pattern thatdefines the at least one character; (b) forming a photoresist layer onthe surface of one of the plate members; (c) exposing all of thephotoresist layer but the blank region to a radiation by using thephotomask of the first type; and (d) forming a latent image of the atleast one character in the blank region of the photoresist layer byusing at least one of the photomasks of the second type.
 2. The methodof claim 1, wherein each said identifier is a character string includingat least two characters, and wherein the step (d) includes the steps of:(d1) performing a first exposure process to form the latent image of oneof the at least two characters in a first portion of the blank region ofthe photoresist layer by using one of the photomasks of the second type;and (d2) performing a second exposure process to form the latent imageof another or the other one of the at least two characters in a secondportion of the blank region of the photoresist layer by using thephotomask of the second type that is the same as, or different from, thephotomask used in the step (d1), thereby forming the character stringincluding the at least two characters in the photoresist layer.
 3. Amethod of recording mutually different identifiers on multiple platemembers, each said identifier including a group of character stringshaving character strings A and B, the character string A including anumber z₁ of characters, the character string B including a number z₂ ofcharacters, where z₁ and z₂ are both natural numbers, the methodcomprising the steps of: (a) preparing a photomask of a first type andat least one photomask of a second type, the photomask of the first typehaving opaque patterns that define a blank region, on which thecharacter string B should be written, and the character string A, the atleast one photomask of the second type having an opaque pattern thatdefines at least one of the characters of the character string B; (b)forming a photoresist layer on the surface of one of the plate members;(c) selectively exposing the photoresist layer to a radiation by usingthe photomask of the first type, thereby defining an unexposed region,corresponding to the blank region, and a latent image of the characterstring A in the photoresist layer; and (d) forming a latent image of theat least one character of the character string B in the unexposed regionof the photoresist layer by using the at least one photomask of thesecond type.
 4. The method of claim 3, wherein the character string Bincludes at least two characters, and wherein the step (d) includes thestep of forming the latent image of one of the at least two charactersof the character string B after another in the unexposed region of thephotoresist layer.
 5. The method of claim 3, wherein the step (d)includes the step of forming the latent image of one of the charactersof the character string B after another in the unexposed region of thephotoresist layer by using a plurality of photomasks of the second type,which includes the at least one photomask of the second type, one byone, the plurality of photomasks of the second type having opaquepatterns that define mutually different types of characters.
 6. Themethod of claim 3, wherein the step (d) includes the step of forming thelatent image of one of the characters of the character string B afteranother in the unexposed region of the photoresist layer by using the atleast one photomask of the second type having opaque patterns thatdefine multiple types of characters on the same plane.
 7. The method ofclaim 3, wherein the step (b) includes the step of forming thephotoresist layer of a negative photoresist material on the surface ofthe plate member, and wherein the method further comprises the step ofdeveloping the photoresist layer of the negative photoresist materialand removing portions of the photoresist layer, which correspond to thecharacter strings, from the surface of the plate member.
 8. The methodof claim 3, wherein the step (b) includes the step of forming thephotoresist layer of a positive photoresist material on the surface ofthe plate member, and wherein the method further comprises the step ofdeveloping the photoresist layer of the positive photoresist materialand leaving portions of the photoresist layer, which correspond to thecharacter strings, on the surface of the plate member.
 9. A method ofrecording mutually different identifiers on multiple plate members, eachsaid identifier including a group of character strings having characterstrings A and B, the character string A including a number z₁ ofcharacters, the character string B including a number z₂ of characters,where z₁ is an integer that is equal to or greater than zero and z₂ is anatural number, the method comprising the steps of: (a) preparingmultiple photomasks of a first type and multiple photomasks of a secondtype, wherein multiple opaque patterns, defining blank regions on eachof which the character string B should be written and the characterstrings A, are arranged periodically in X-axis and Y-axis directions oneach of the photomasks of the first type, and wherein multiple opaquepatterns, each defining one of the characters of the character string B,are arranged periodically in the X-axis and Y-axis directions on each ofsaid photomask of the second type; (b) forming a photoresist layer onthe surface of one of the plate members; (c) selectively exposing thephotoresist layer to a radiation by using one of the photomasks of thefirst type, thereby defining unexposed regions, corresponding to theblank regions, and latent images of the character strings A in thephotoresist layer; and (d) forming latent images of one of thecharacters of the character string B after another in the unexposedregions of the photoresist layer by sequentially selecting a requiredone of the photomasks of the second type after another.
 10. The methodof claim 9, wherein the step (a) includes the step of arranging mdifferent types of characters of the character strings B within aportion of at least one of the photomasks of the second type, theportion having a length of y2 as measured in the Y-axis direction, wherey2 is a pitch between two characters of the same type on each saidphotomask of the second type, and wherein an inequality m≦y2/y1 issatisfied, where y1 is a size of the characters as measured in theY-axis direction on each said photomask of the second type.
 11. Themethod of claim 9, wherein the step (d) includes the steps of: (d1)aligning a first one of the photomasks of the second type with thephotoresist layer so that a first selected character on the firstphotomask of the second type is projected onto an associated portion ofthe unexposed region; (d2) performing a first exposure process to form alatent image of the first selected character in the unexposed region ofthe photoresist layer after the step (d1) of aligning the firstphotomask of the second type with the photoresist layer has beenperformed; (d3) aligning one of the first photomask of the second typeand a second one of the photomasks of the second type with thephotoresist layer so that a second selected character on the first orsecond photomask of the second type is projected onto another associatedportion of the unexposed region; and (d4) performing a second exposureprocess to form a latent image of the second selected character in theunexposed region of the photoresist layer after the step (d3) ofaligning the first or second photomask of the second type with thephotoresist layer has been performed.
 12. The method of claim 11,further comprising the step of forming alignment marks before the step(b) of forming the photoresist layer is performed, wherein at least oneof the step (d1) and the step (d3) is carried out by using the alignmentmarks.
 13. The method of claim 9, wherein the steps (c) and (d) includethe step of forming the latent images of the character strings over theentire surface of the photoresist layer by using a mask aligner.
 14. Themethod of claim 9, wherein the steps (c) and (d) include the step offorming the latent images of one of the character strings after anotherin multiple divided regions of the photoresist layer by using a stepper.15. A method of recording mutually different identifiers on multipleplate members, each said identifier including a group of characterstrings having character strings A, B and C, the character string Aincluding a number z₁ of characters, the character string B including anumber z₂ of characters, the character string C including a number z₃ ofcharacters, where z₁ is an integer that is equal to or greater than zeroand z₂ and z₃ are both natural numbers, the method comprising the stepsof: (a) forming a photoresist layer on the surface of one of the platemembers; (b) performing a first exposure process in such a manner thatlatent images of the character strings A and C are formed in each ofmultiple subdivided regions of the photoresist layer but such that ablank region of each said subdivided region of the photoresist layer, onwhich the character string B should be written, is not exposed to aradiation; and (c) performing a second exposure process in such a mannerthat latent images of one of the characters of the character string Bare formed in the blank regions of the multiple subdivided regions ofthe photoresist layer and then latent images of another character of thecharacter string B are formed in the blank regions.
 16. The method ofclaim 15, wherein the step (b) includes the step of forming the latentimages of mutually different character strings C in the multiplesubdivided regions of the photoresist layer.
 17. The method of claim 16,wherein the multiple subdivided regions of the photoresist layer arearranged in columns and rows, and wherein supposing the character stringC that is allocated to one of the multiple subdivided regions is locatedat an intersection between an M^(th) row and an N^(th) column (where Mand N are both natural numbers) and is identified by C_(MN), M≠J and/orN≠K (where J and K are both natural numbers), C_(MN)≠C_(JK) issatisfied.
 18. The method of claim 17, wherein supposing the characterstring A allocated to one of the multiple subdivided regions that islocated at the intersection between the M^(th) row and the N^(th) column(where M and N are both natural numbers) is identified by A_(MN), M≠Jand/or N≠K (where J and K are both natural numbers), A_(MN)=A_(JK) issatisfied.
 19. The method of claim 15, wherein the step (b) ofperforming the first exposure process includes the step of exposing thephotoresist layer to the radiation by using at least one photomask of afirst type having opaque patterns that define the character strings Aand C, and wherein the step (c) of performing the second exposureprocess includes the step of exposing the photoresist layer to theradiation by using at least one photomask of a second type having opaquepatterns that define the characters of the character string B.
 20. Themethod of claim 15, wherein the step (b) of performing the firstexposure process includes the step of exposing the photoresist layer tothe radiation by using at least one photomask of a first type havingopaque patterns that define the character strings A and C and opaquepatterns that define unexposed regions for the character strings B, andwherein the step (c) of performing the second exposure process includesthe step of exposing the photoresist layer to the radiation by using atleast one photomask of a second type having opaque patterns that definethe characters of the character strings B.
 21. The method of claim 15,wherein the step (b) of performing the first exposure process includesthe step of forming the latent images of the character strings A and Cover the entire surface of the photoresist layer by using a maskaligner.
 22. The method of claim 15, wherein the step (c) of performingthe second exposure process includes the step of forming the latentimages of the character strings B in one of multiple divided regions ofthe photoresist layer after another by using a stepper, each of themultiple divided regions of the photoresist layer being wider than eachof the multiple subdivided regions of the photoresist layer andincluding the multiple subdivided regions of the photoresist layer. 23.The method of claim 22, wherein the step of forming the latent images ofthe character strings B includes the step of forming the latent imagesof one type of character strings B in one of the divided regions of thephotoresist layer and then forming the latent images of another type ofcharacter strings B in another one of the divided regions of thephotoresist layer.
 24. A method of patterning a photoresist layer, themethod comprising the steps of: preparing the plate member including thephotoresist layer that has been exposed to the radiation by the methodas recited in claim 1; and developing the photoresist layer.
 25. Amethod of recording an identifier on a plate member, the methodcomprising the steps of: preparing the plate member including thephotoresist layer that has been patterned by the method as recited inclaim 24; and transferring the pattern of the identifier onto thesurface of the plate member by using the patterned photoresist layer asa mask.
 26. The method of claim 25, wherein the step of transferring thepattern of the identifier includes the step of selectively etching awaysurface portions of the plate member that are not covered with thepatterned photoresist layer.
 27. The method of claim 25, wherein thestep of transferring the pattern of the identifier includes the step offorming convex portions on surface regions of the plate member that arenot covered with the patterned photoresist layer.
 28. The method ofclaim 25, wherein the step of transferring the pattern of the identifierincludes the step of altering surface regions of the plate member thatare not covered with the patterned photoresist layer by exposing thesurface regions to an energy beam.
 29. A method of manufacturingelectronic components, the method comprising the steps of: preparing theplate member on which the identifiers have been recorded by the methodas recited in claim 25; depositing a thin film on a selected surface ofthe plate member; and dicing the plate member into multiple dividedelements.
 30. The method of claim 29, further comprising the step ofrecording the identifiers on the plate member so that each said dividedelement has one of the identifiers on the surface thereof.
 31. Themethod of claim 30, wherein the step of recording the identifiersincludes the step of recording the identifiers on the plate member sothat each said divided element has a unique identifier on the surfacethereof.
 32. A set of photomasks for use to transfer the pattern of anidentifier, including a character string that includes at least twocharacters, onto a photoresist layer, the set comprising: a photomask ofa first type including an opaque pattern that defines a blank region onwhich the character string should be written; and at least one photomaskof a second type including opaque patterns that define the at least twocharacters of the character string.
 33. A set of photomasks for use totransfer the pattern of an identifier onto a photoresist layer, theidentifier including a group of character strings having characterstrings A and B, the character string A including a number z₁ ofcharacters, the character string B including a number z₂ of characters,where z₁ is an integer that is equal to or greater than zero and z₂ is anatural number, the set comprising: a plurality of photomasks of a firsttype, wherein multiple opaque patterns, defining blank regions on eachof which the character string B should be written and the characterstrings A, are arranged periodically in X-axis and Y-axis directions oneach said photomask of the first type; and a plurality of photomasks ofa second type, wherein multiple opaque patterns, each defining one ofthe characters of the character string B, are arranged periodically inthe X-axis and Y-axis directions on each said photomask of the secondtype.
 34. The set of photomasks of claim 33, wherein m different typesof characters of the character strings B are arranged within a portionof at least one of the photomasks of the second type, the portion havinga length of y2 as measured in the Y-axis direction, where y2 is a pitchbetween two characters of the same type on each said photomask of thesecond type, and wherein an inequality m≦y2/y1 is satisfied, where y1 isa size of the characters as measured in the Y-axis direction on eachsaid photomask of the second type.
 35. The set of photomasks of claim33, wherein multiple opaque patterns, defining character strings C, arearranged periodically in the X-axis and Y-axis directions on each saidphotomask of the first type, each said character string C including anumber z₃ of characters, where z₃ is a natural number.